The present invention relates generally to computer networks and more particularly to computer networks of the type in which a plurality of geographically dispersed computer stations are interconnected for interstation communications by a single bi-directional bus.
In recent years a number of different types of computer networks have been proposed and in some cases actually implemented wherein a plurality of geographically dispersed computer stations are interconnected either for communication purposes or for collectively performing system tasks from different locations or for sharing a data storage facility at one of the computer stations. If the network is one in which computer stations are relatively close, that is, within around one kilometer from each other, the network is often called a local computer network.
In one type of local computer network, the computer stations are interconnected by a single bi-directional bus which is used by the stations in a message or packet switching mode. Some of the advantages of the single bi-directional bus type network are its low cost, its overall simplicity and its capability for easily adding or removing computer stations when desired or required. In the single bus bi-directional network, all station-to-station communications, including status and control signals, take place over the bus. A message can be sent by any computer station connected to the bus and can be received by every other computer station connected to the bus. Only one message, however, can be transmitted over the bus at one time. If two or more computer stations attempt to send messages over the bus at the same time, the messages will collide and be lost. In addition, the colliding messages may create a new message which is different from all of the original messages. The new message may be received by an intended recipient of one of the original messages or even by a computer station that was not intended to be a recipient of any of the original messages. Therefore, in the operation of a single bi-directional bus network it is necessary that access to the bus by the computer stations be controlled.
A number of different techniques have been devised for controlling access to the bus in a single bi-directional bus network. As far as is known, however, no technique has been devised which is completely reliable as far as performance is concerned.
In one prior art technique, known as the selection technique, a computer station may access the bus only when it has been signalled that it is its turn to do. In one class of networks using this technique, the signals placing the computer station in control are generated by a central controller unit and then sent to the various computer stations either by a daisy chaining arrangement, by a polling arrangement or by an arrangement known as independent requests. In another class of networks using this technique, there is no central controller unit. Instead, the control logic is distributed evenly among the computer stations. The control signals which are generated by the computer stations are set from one to the other by daisy chaining, by polling or by independent requests. The main problem with the selection technique for controlling access is that the network is completely dependent on the operation of one unit. If the network includes a central controller unit and the central controller unit fails, the network is inoperative. If the network does not contain a central controller unit and the computer station in temporary control fails, the network is inoperative.
In another prior art technique known as the random access technique, a computer station desiring to access the bus does not have to wait until it is placed in control but simply monitors the bus for activity. If there is activity, the computer station waits. If there is no activity for a predetermined time interval, the computer station assumes the bus is clear and transmits its message. The problem with this technique is that, because of the axiomatic relationship between time and space that "one cannot be in more than one place at a given point in time", two (or more) computer stations can arrive at the same conclusion that the bus is clear at approximately the same time and access the bus simultaneously. When this happens, the two messages will collide and be lost.
In still another prior art technique known as the reservation technique, a computer station desiring to transmit a message places a request to do so and then receives a future reserved time during which it may transmit its message. The main problems with this technique are that it is dependent on the operation of the unit through which the requests must be made and is relatively slow.
As can be appreciated, the selection technique suffers in that the network operation is dependent on one unit for continuous operation, the random access technique suffers in that it is subject to collision situations and the reservation technique suffers in that it is relatively slow and that the network is dependent on one unit for continuous operation.
A more detailed discussion of the above techniques may be found in an article entitled Global Bus Computer Communication Techniques by E. C. Luczak appearing in the 1978 I.E.E.E.
For many proposed or actual applications, a single bus network is needed which does not merely eliminate or reduce one shortcoming at the expense of creating another but rather does not possess any of the aforementioned problems or limitations. The present invention provides such a network.